107 



213. Uses. — The automatic gage is especially useful for the estab- 

 lishment of tidal datum planes, for securing the data for the harmonic 

 analysis of the tides, and for hydrographic surveying when it is not 

 possible to establish a gage within sight of the area surveyed. The 

 staff gage is usually more convenient for determining the varying 

 elevation of the water during surveys made in the vicinity of the 

 gage, and for regulating the operation of dredges, since the elevations 

 are immediately available and can be read from a distance. For 

 establishing a low water datum by comparison, only the heights of 

 the high and low waters need be taken off the record; although the 

 times of their occurrence may be taken off also as a check, or to 

 determine the luni tidal intervals, if a determination of the latter is 

 desired. 



214. Reference to bench marks. — A staff gage is easily destroyed and 

 usually lasts for but a short time, unless at least it is built into a per- 

 manent structure. Even in the latter case the structure may settle 

 or suffer enough disintegration to displace the zero of the gage. The 

 record of an automatic gage is dependent on its accompanying staff 

 gage. No tide gage serves much useful purpose, therefore, unless its 

 zero is referred to stable shore bench marks, and if a valuable record 

 is desired it should be referred to at least three bench marks well 

 separated from each other. Staff gages for surveys and for the opera- 

 tion of dredges are ordinarily set from bench marks, with their zero 

 at the established datum. 



215. Operation of an automatic gage. — No clock keeps perfect time, 

 and a clock mechanism driving a relatively heavy recording device 

 cannot be expected to. The registering apparatus of the gage may 

 bend or lag or get out of order, the intake and the well may clog, the 

 float may leak and the wharf or other structure on which the gage is 

 installed may settle. An automatic gage must therefore be tended 

 daily to see that it is functioning properly, and the height of the tide 

 on the staff, with the time at which it is taken, inscribed on the record 

 of the gage. The gage must be inspected by an engineer at intervals, 

 and the zero of the staff gage checked against the reference bench 

 marks. The detailed technique for the installation and operation of 

 automatic tide gages and the tabulation of the record, is given in a 

 Manual of Tide Observation, Special Publication 196, United States 

 Coast and Geodetic Survey. Because of the cost of securing them, 

 reliable records of the tide over any considerable period are available 

 only at a relatively limited number of stations in the harbors of the 

 United States; but these are sufficient to afford a good determination, 

 by comparison, of the datums at any point on the coast line. 



192750—40 8 



